The invention relates in general to respirators and in particular to a new and useful piston-cylinder unit employed as a respiratory gas supply unit which can be actuated by means of an actuation device for the respiratory gas in the respiratory cycle of the respirator.
Actuation units in various executions are used for the transport of the respiratory gas in the respiratory cycle of a respirator, a well-known kind of actuation device is described in the monograph: "Automatic Ventilation of the Lungs", William W. Mushin et al., Blackwell Scientific Publications, 3rd edition, pages 273 to 277. Herein the diagram of a respirator is shown in which the respiratory air is transported in the respiratory cycle by means of a piston-cylinder unit. The outer surface of the piston is sealed off against the inner wall of the cylinder by means of two rolling membranes which can follow the piston movement in both directions in their fold arranged between the piston and the cylinder. The rolling membranes separate the cylinder chamber into a working chamber for the generation of the pressure required for the artificial respiration and an antechamber in which the air is taken in during the working stroke to generate a vacuum during the expiration. The actuation device is an electro-motor with a gear and levers for the transmission of the rotating movement into a lifting movement. During the artificial respiration the piston executes alternating stroke movements for the generation of inspiration-phases and expiration-phases, whose frequency and lift of stroke can be varied by means of a gear and lifting rod. Therefore, depending on the control of the piston, the rolling membranes execute a shorter or longer lift of stroke, during which the respective fold of the rolling membrane rolls off in the space between the piston and the inner surface of the cylinder, following the lift of the stroke.
In an arrangement of the rolling membrane according to the known piston-cylinder unit it is disadvantageous, that because of the changing pressure conditions in the antechamber as well as in the working chamber the capability of the membrane material to roll off becomes irregular and unpredictable in the proximity of the fold and may even fail when the membrane material sticks to itself in the fold. This danger is given particularly during the starting of the piston, as no respiratory pressure has built up yet. Such an unnoticed wear easily leads to leaks in the material, and therefore to pressure loss and gas loss. This can have serious consequences in nowadays precisely dosed amounts of gas in respiration methods with less and less gas surplus and a respiratory cycle which is practically a closed cycle.